This invention relates to the mounting of photographs, posters, digital ink jet prints, and fabrics to a rigid board for display purposes in the picture framing, sign and display markets. These types of boards are commonly known as heat activated foam boards, or HA boards.
One of these kinds of display items can be placed on the heat activated adhesive on the board and sent through a heated roll laminator or a flat press using heat and pressure created from either a mechanical system or a vacuum mechanism.
For many decades various means to adhesively attach display items to a foam board have been employed. Prior to boards having a direct coating of adhesive on one side of the board there were adhesive coated papers or free adhesive films known in the art as dry mount tissues used to adhere items such as photographs to a composite foam board. These adhesive tissues are still used today and present various problems with their application. These tissues are coated on light weight paper (20#) and have lay flat issues due to moisture absorption and coating methods. Having to cut to size these adhesive tissues is quite timely and creates greater risk of air entrapment during mounting now that there are three separate items to bond together.
Entrapped air causes visible bubbles and additionally, dirt particles picked up from manufacturing or storage show up in the final mount as bumps and generate major complaints from customers. The challenge is how to keep the adhesive surface clean and eliminate air bubbles. Many attempts over the years have not eliminated this problem.
Most of these tissues require high temperatures to initiate a bond to the foam board and the item to be mounted. The most popular tissue being sold today goes under the trade name Colormount. This product from D&K Group Inc. has a recommended application temperature of 175-205 F. At these temperatures many photographs and other heat sensitive items will be negatively affected. The foam of the board itself will also begin to change its physical appearance at these temperatures. When using dry mount tissues it is also recommended by these suppliers that one buys their heavy metal plate to place the still hot mounted board with the newly mounted item under the weight of the plate to allow the package to cool so the corners of the photographs or display items will not lift off the warm adhesive. These tissues are rather costly with some in the 0.45 cents per square foot range.
A few years after the introduction of dry mount adhesive tissues manufacturers began to direct coat a heat activated adhesive one side of the foam board to replace the dry mount tissue. New equipment in the format of hot roll laminators became popular for the laminating and mounting of these boards due to their small physical footprint in a shop and their speed. The traditional small format mechanical press and the larger vacuum press were the main equipment used in the industry for mounting and laminating for many decades but had job size limitations and were slow in production.
The most widely used coated board in the market is Encore Single Step by Elmers Products, with a recommended application temperature of 180 F. This temperature is too hot for both the foam center core and many items that may needed to be mounted. This mounting board and many others do not have universal adhesion to all the types of items that may need to be mounted. The Elmers Single Step product literature includes a disclaimer that this product is not recommended for RC (resin coated) photo papers, a very important type of item for mounting. Some companies have come out in the market with low temperature coated foam board in the range of 160 F to get around the issue of excessive heat creating cosmetic defects for the foam, photographs and other mounting items. It appears the lowered application temperature of the adhesive also decreases the hot tack, with the consequence that the ultimate bond is decreased and corners frequently lift off the board immediately after mounting.
Another problem with the lower activation adhesive coated boards when used with a hot roll laminator for mounting is adhesive transfer. It is customary when sending a job through the laminator some of the top adhesive layer is exposed and not covered up by the poster or mounting item and this adhesive will come into direct contact with the heated silicon rubber roll and transfer to the roll. Because silicon rubber is release material the roll transfers the adhesive onto the surface of the next job and renders it unacceptable.
There are two basic methods for getting the adhesive on the foam board products surface. The adhesive may be extruded from pellets with products like EVA or EMA resins into a smooth adhesive coating on the foam board surface or they may be water based dispersions of PVA coated and dried onto the foam board surface. Current water based PVA products have a very pronounced coating pattern evident on the dried adhesive, which looks like roll splatter. This is evident in the mounted print surface, reducing the quality of the final product. Current EVA extruded coatings on boards are smooth but have the drawbacks of adhesive transfer to the hot laminating rolls and insufficiently low activation temperature to avoid all the heat-related problems discussed above.
It must be noted that trying to lower the melting points or activation temperature of the EVA adhesives also makes them transfer more to the hot rubber laminator rolls. So conventional thinking in this industry is that extruded EVA cannot provide a total solution.
It must also be noted that a desired smooth surface adhesive coated board will result in air bubbles and ruined jobs when used in a heated vacuum or mechanical platen press due to a non-porous item (i.e., resin coated photo paper) mounted to a non-porous coated board. The air between the item to be mounted and the board cannot all be displaced simultaneously over a large surface areas, and cannot pass through the mating surfaces, so entrapment of some air is inevitable. The aqueous coated PVA boards rely on the coating roll texture to help create a rough surface on the adhesive that provides air channels for entrapped air to escape during the mounting process. As mentioned above this solution to air bubble entrapment exhibits the drawback of undesirable surface texture in the final product.
In a production environment, “time is money”, and it is therefore critical how long it takes to send a board through a hot roll laminator or heated vacuum press. Roll laminators are quicker and take up less floor space and can handle larger format pieces than either the mechanical press or the vacuum press. Industry standards for the vacuum and mechanical press are on average 2-3 minutes duration at a temperature of 160 F-180 F for good adhesion to the adhesive coated board. The speed for the hot roll laminator for Elmers Single Step, a very popular heat activated product, is 3 feet per minute at a roll temperature setting of 250 F. Three feet per minute is considered a fast speed in this industry and at faster speeds adhesion suffers.
It can thus be appreciated that the objective in the HA foam board mounting industry is to produce quality mounting using the lowest temperature at the highest speed.
This wish list was partially addressed with the introduction of pressure sensitive adhesive (PSA) coated boards taking the heat out of the mounting process with a cold seal through a cold roll laminator. Certainly taking the temperature out of the lamination makes for less wrinkling of the materials to be mounted, since heated rolls drive moisture out of paper based items and heat distorts plastic mounting items. However a new set of issues become evident with these PSA boards. When the release liner is removed, the adhesive surface is very tacky and aggressive requires a great deal of care so as to not let anything touch the surface before the board is fed to the laminator. Although the adhesive bond may look good there are some risks that over time a bond can let go from the adhesive due to moisture absorption and paper mounted items changing in dimension and exhibiting bumps and ridges between the foam board and mounted items such as posters or photos.
PSA boards are not considered as providing as good a bond to some mounted items as heat activated boards. PSA boards are often used with photographs even though it is much more difficult to master the mechanics of the process when compared to a heat activated process. Many of the heat activated boards will not bond to the difficult photo papers and the paper's chemistry is affected by heat in a detrimental manner so the fall-back option, although a challenging one, has been to use a PSA coated board for this application.
It is quite evident that although many types of equipment and foam boards have been used in this industry for decades, no universal solution has been found to overcome the problems of slow process throughput, high temperature, and surface irregularity in the final product.